The common way to do it is using a random number generator, in the case of GDScript, "randf()" is the best for this.
With that function you will get a random number between 0 and 1, you can take 0 as 0% and 1 as 100%.
That way, dividing the range 0 to 1 in percentile-like fashion you can make decisions according the result of the generator.
Example: if you have 30% chances, check if the random number is
> 0.7) .
There is a probability of 30% that the randf result in a number <= 0.3 (or > 0.7).
ps: remember to use
randomize() somewhere before the use of random number generators.